Currently Fivetran's Slack app requires the app to be manually joined to the channel.
However, this makes it difficult to analyze the entire Slack message if you want to do so.
For example, if you already have many existing channels, it would be impractical to manually join all of them.
Or, when a new channel is created, it is not possible to capture the interactions on that channel.
Fortunately, Slack's API allows you to add apps to all public channels, or to automatically join new channels as they are created.
Use the Fivetran app home screen to activate the feature.
You could set up a button to join all channels.
You should be able to install a feature to automatically join the app when a public channel is created. (I have created several Slack apps and have used them successfully that way)
Please consider.